The isomerization reaction of n-alkanes is an important reaction in the petroleum processing process, such as the isomerization dewaxing process of lube oil and the isomerization visbreaking process of diesel oil. N-alkanes are non-ideal constituents in lube distillates, and may result in increased pour point and degraded low-temperature properties of the lube distillates. To improve those properties, the n-alkanes, which have high melting points, must be removed fully or partially. At present, common dewaxing methods mainly include solvent dewaxing, catalytic dewaxing, and isomerization dewaxing. Solvent dewaxing is to utilize the solubility of wax in a solvent to remove wax, but has the following drawbacks: it is difficult to choose an appropriate solvent, a large amount of organic solvent is wasted, the treatment process is harmful to human body and may pollute the environment, the equipment investment and operation costs are high, and the product quality is confined by the raw material. Catalytic dewaxing is to utilize a catalyst that has a shape-selective cracking function to carry out selective catalytic cracking of the wax constituent in the distillate, so that the wax constituent is cracked into hydrocarbons with smaller molecules. For example, in the U.S. Pat. No. 4,247,388, a catalytic dewaxing method is used to remove wax from lube oil. The drawbacks of the catalytic dewaxing method include: the base oil yield is low, the loss of viscosity index is severe, and the values of the byproducts are low, since a large quantity of high-value macromolecular compounds are converted into low-value substances with small molecules. Compared with the two dewaxing method described above, isomerization dewaxing is to make macromolecular wax have an isomerization reaction to generate isoalkanes, which have lower solidifying points and pour points and higher viscosity indexes than the wax that has the same molecular weight, and the isoalkanes still remain in the heavy distillate; thus, the yield of distillates is greatly improved.
Presently, there are many reports on isomerization dewaxing catalysts. For example, the U.S. Pat. Nos. 5,990,371, 5,833,837, 5,817,907, 5,149,421, 5,135,638, 5,110,445, 4,919,788, 4,419,420, 4,601,993, 4,599,162, and 4,518,485, etc. relate to isomerization dewaxing techniques. The acidic constituents used in those isomerization dewaxing techniques mainly include mordenite and molecular sieves SAPO-11, SAPO-31, SAPO-41, ZSM-23, SSZ-32 and ZSM-48. Molecular sieves different from each other in structure are suitable for different purposes, because they have unique pore canal structures and physical and chemical properties. Molecular sieve ZSM-5 has pore canals in three-dimensional ten-membered ring structure, molecular sieve NU-87 has pore canals in two-dimensional ten-membered ring structure, molecular sieves ZSM-22, EU-1 and SAPO-11 have pore canals in one-dimensional ten-membered ring structure, and molecular sieve MCM-22 has two separate multi-membered ring pore canal systems, wherein, one system has two-dimensional sinusoidal and crossed ten-membered ring pore canals. Those molecular sieves have their unique effects in isomerization reactions of hydrocarbon molecules. However, the effects are somewhat different from each other when those molecular sieves are used in isomerization dewaxing, owing to the slight differences in the pore canal structures and physical and chemical properties. All of those materials can make paraffinic hydrocarbons have isomerization reactions to some extent. However, for some reactions, e.g., reactions of lube distillate, the reactions are not always participated by the same molecules, because the constituents are complex. Therefore, molecular sieves that have a uniform structure may have some adaptive problems in those reactions. Consequently, a catalytic material that matches the molecules of one constituent well may be not ideal for other constituents. The n-alkanes in lube distillate and diesel oil result in degraded low-temperature fluidity of lube oil and diesel oil, owing to their high solidifying points. Isomerization dewaxing is to utilize a bi-functional catalyst with special pore structures to convert long-chain n-alkanes and polycyclic naphthenes that have high melting points in the wax constituent into isoalkanes with single branched chain and monocyclic naphthenes with long branched chains that have lower melting points. However, the melting point of the alkane will be increased if the isomerization degree is too high. Therefore, the isomerization degree of the wax molecules must be controlled appropriately. Hence, there is a strict requirement for the acidic property and pore structure of the acidic constituent and the hydrogenation constituent. Generally speaking, the acidic constituent(component) should have acid sites with moderate intensity and pore structures with space limitation function; in addition, the active metal constituent must have activity for rapid hydrogenation/dehydrogenation, so as to prevent further isomerization or even cracking of tertiary carbonium ions. Since the cracking reaction converts high-carbon alkanes into small-molecular low-carbon alkanes and thereby decreases the yield of the target product, the isomerization reaction should be promoted as far as possible, and the cracking reaction should be inhibited as far as possible. The patent document U.S. Pat. No. 6,204,426 has put forward a process for preparing an isomerization catalyst utilizing a mixture of SAPO-11, SAPO-31, SAPO-41, ZSM-48, ZSM-23 and ZSM-35, etc. as a carrier, and the patent document U.S. Pat. No. 5,833,837 has put forward a process for producing lube base oil with a dual-catalyst system, i.e., using SAPO-11, SAPO-31, and SAPO-41 series catalysts for isomerization of light lube oil constituents respectively, and using ZSM-5 catalyst for isomerization of heavy lube oil constituents. However, both of the processes have their drawbacks. Specifically, the former process can't give full play to the synergetic effect of the constituents of the mixed molecular sieve, and the latter process prolongs the process flow, increases operating difficulties, and severely increases the cost of investment.